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A287221
Number of twice-crossing partitions on n nodes.
2
0, 0, 0, 0, 8, 42, 168, 760, 2418, 10490, 30842, 131676
OFFSET
0,5
COMMENTS
The meandric numbers A005316 are the numbers of paths which cross themselves 0 times.
This sequence is the number of paths that must cross themselves exactly twice.
EXAMPLE
a(4) = 8; this is from the partitions
(2,1,4,3), (2,4,3,1),
(3,2,4,1), (3,4,2,1),
(4,1,3,2), (4,2,1,3),
(4,2,3,1), (4,3,1,2).
CROSSREFS
Sequence in context: A319235 A341764 A256861 * A119965 A229729 A055082
KEYWORD
nonn,more
AUTHOR
STATUS
approved